1. Field of the Invention
The present invention relates to an exposure system and an exposure method, more particularly to an exposure system and exposure method controlling an incidence angle of charged particles to a mask arranged facing a wafer to correct a position of a transfer patter in accordance with distortion of an underlying pattern of the wafer.
2. Description of the Related Art
Japanese Unexamined Patent Publication (Kokai) No. 11-135423 discloses low energy electron beam proximity projection lithography (LEEPL), which brings a stencil mask into proximity with a wafer for exposure, using an electron beam sub deflection function to control the direction of an electron beam (EB) to correct offset between a pattern of the mask and an underlying pattern on the wafer.
Japanese Unexamined Patent Publication (Kokai) No. 2003-59819 discloses complementary exposure enabling exposure of a donut shaped pattern or a leaf pattern by a stencil mask by dividing that pattern of the mask into two or more parts. To perform complementary exposure efficiently, the complementary patterns are formed adjacent to each other on the mask and exposed at one time.
If an underlying pattern of a wafer has drastic distortion at an area narrower than the EB diameter, when correcting distortion between the underlying pattern and a transfer pattern by electron beam sub-deflection, a low pass filter effect caused by the electron beam intensity distribution causes insufficient correction and residual distortions. Such drastic distortion occurs due to the magnification, rotation, or offset of the center of gravity of the chip and has to be corrected by electron beam sub-deflection. In complementary exposure, insufficient correction is caused by the electron beam being simultaneously focused on the chips at the two sides of a strut.
For decreasing the residual distortion remaining even with the electron beam sub-deflection function, it is effect to make the EB diameter smaller. However, due to following reasons, the EB diameter is difficult to make smaller.
LEEPL uses as a current source a cathode formed by LaB6. The electrons emitted from the cathode radiate from points on the surface of the cathode with a spreading angle. If the amount of current emitted from a unit area of the surface of the cathode to a unit solid angle is defined as the luminance, the electron beam optical system emitting the electron beam to the mask is maintained at a constant luminance if ignoring Coulomb interaction. Therefore, since the luminance is not increased due to Coulomb interaction, if focusing the electron beam finely, the convergence angle of the electron beam is increased.
However, if the electron beam convergence angle increases, the resolution of a transfer pattern drops. On the other hand, if maintaining the resolution constant and making the electron beam smaller in diameter, the amount of current drops. The exposure time is an important factor determining LEEPL performance. If the electron beam is made low in current, the exposure time becomes longer, so becomes a primary factor behind the drop in throughput. Further, the luminance of a cathode is limited by its material, so making the cathode higher in luminance so as to make up for amount of current and maintain resolution is difficult.
The above discussion concerned the theoretical limits. However, even within the theoretical limits, for maintaining the resolution and the exposure time while making the EB diameter smaller, the electron beam optical system has to be made higher in performance, so the cost increases. If making the cathode larger in size and larger in current to shorten the exposure time and improve the throughput of the exposure system, the EB diameter ends up becoming large due to the unchanging luminance. Therefore, there are limits to the reduction of the EB diameter.